Cellular resistance to anti-cancer agents is the predominant reason why chemotherapy regimens fail to eradicate disseminated malignancies. Therefore, elucidating the resistance mechanisms is of great importance for improving patient outcome. Several mechanisms for drug resistance have been identified including elevated drug pump activity and p53 mutation. In addition, the drug sensitivity of many tumors is influenced by induction of the stress response pathway. Moreover, the pathway that responds to reactive oxygen species (ROS) appears to be of singular importance for influencing drug sensitivity. However, the molecular mechanisms underlying the role of the ROS pathway in altering the drug response are largely unknown. The long-term goal of this proposal is to fully describe the ROS response system and determine how this pathway influences drug sensitivity in both normal and transformed cells. Toward this goal, we have chosen to study the relationship between the oxidative stress pathway and drug response in the budding yeast S. cerevisiae. Previous studies from this laboratory have identified two critical regulators that control both the oxidative stress-induced signal transduction pathway and gene expression. Ume3p is the yeast C-type cyclin that, rather than control the cell cycle, represses the transcription of several stress response genes. To relieve this repression, Ume3p is destroyed in cells exposed to oxidative stress. This destruction requires the conserved signaling molecule phosphatidylinositol-specific phospholipase C (PLC1). The importance of Plc1p-directed destruction of Ume3p is underscored by the finding that deleting the cyclin can rescue plc1 mutants from stress-induced cell death. The conservation of this pathway is supported by two findings. First, Plc1p is most similar to mammalian Plc-gamma, which also transduces an oxidative stress signal. In addition, the human cyclin C is also destroyed in response to stress in both human and yeast cells. The proposal combines the powerful genetic and molecular tools available in yeast to dissect the oxidative stress signaling pathway and determine its nuclear targets. We will combine our genetic system with microarray analysis to pinpoint genes whose expression is essential for viability in response to ROS. Finally, these genes will be ectopically activated or deleted in both yeast and human cells to determine their role in drug susceptibility or resistance.